Solid state overload relay with phase unbalance protection having RMS current approximation

Abstract

A solid state overload relay utilizes an apparatus that converts the current of a polyphase electrical load to a voltage signal approximately proportional to the RMS current of the load. In addition to providing the overload function, the voltage signal is used to provide an improved phase current unbalance protection. Conventional current transformers sense the current flow in each phase of the load. The current waveform for each phase is converted to a voltage by rectification and the peak value and the average value of the resultant voltage is determined. A summing circuit combines the two values at a predetermined ratio to create a DC voltage that is approximately equal to the rms value of the input phase current. A combination of the approximation circuit with a time integrating circuit and a level detecting circuit produces a low cost solid state overload relay in one embodiment of the invention. The phase current unbalance circuit determines the average of the DC voltages from each phase and compares this average with the DC voltage from each individual phase. It will output a trip signal if any of the individual phase currents exceeds the average by a predetermined percentage of the average for a set time interval. The improved device does not require compensation for non-sinusoidal currents, line harmonics, or transients which could cause erroneous results.

I claim: 
     
       1. An apparatus for detecting AC current line unbalance between phases of a polyphase AC source supplying power to a load comprising: a. a current sensor in each phase to sense actual current supplied to said load from said polyphase AC source;   b. an approximation circuit in each phase including a converter to convert said actual current to a first voltage proportional to said actual current and a first amplifier to generate a second voltage from said first voltage, said second voltage proportional to an approximate value equal to the rms current in said phase supplied to said load;   c. an averaging circuit to sum said second voltage from each phase and to determine an average value that is an average of the second voltage of all said phases, said average value approximately equal to an average rms current in each phase of said polyphase AC source, said averaging circuit including means to scale said average value by a predetermined factor representative of an allowable level of phase current unbalance between each phase of said polyphase AC source, said averaging circuit to generate a third voltage level, said third voltage level proportional to said allowable level of phase current unbalance;   d. a comparator in each phase to compare said third voltage level with said second voltage proportional to the approximate rms current of said phase, said comparator to output a signal to indicate a phase current unbalance condition if said second voltage exceeds said third voltage scaled algebraic sum; and   e. a time integrator to receive said output signal from said comparator in each phase of said polyphase AC source, and said time integrator to output a trip signal to indicate an unbalance condition if any of said output signals is present for a predetermined time period.   
     
     
       2. The apparatus for detecting AC current line unbalance between phases of a polyphase AC source of claim 1 wherein said approximation circuit in each phase includes first means to generate a voltage from said first voltage proportionally equivalent to an average value of said actual current, second means to generate a voltage from said first voltage proportionally equivalent to a peak value of said actual current, and third means to sum a fraction of said voltage from said first means with a fraction of said voltage from said second means to provide said second voltage proportional to the rms current of said load. 
     
     
       3. The apparatus for detecting AC current line unbalance between phases of a polyphase AC source of claim 2 wherein said approximation circuit solves an approximation equation to provide said output signal proportional to the rms current of said load, said approximation equation is equivalent to the equation   I.sub.rms =(0.22*I.sub.pk)+(0.776*I.sub.ave)     wherein I rms  is the rms current of said load, I pk  is the peak current of said load, and I ave  is the average current of said load.   
     
     
       4. The apparatus for detecting AC current line unbalance between phases of a polyphase AC source of claim 1 wherein said polyphase AC source is a three phase AC source. 
     
     
       5. The apparatus for detecting AC current line unbalance between phases of a polyphase AC source of claim 1 wherein said averaging circuit comprises an operational amplifier having an adjustable resistor in a feedback path between the amplifier output and its inverting input, said resistor for determining gain of the amplifier, said gain to scale said average value by said predetermined factor representative of an allowable level of phase current unbalance between each phase of said polyphase AC source. 
     
     
       6. The apparatus for detecting AC current line unbalance between phases of a polyphase AC source of claim 5 wherein said averaging circuit further includes a voltage divider between an output of said operational amplifier and circuit common, said voltage divider comprising two series connected resistors having a resistance ratio of 2:1 for a three phase AC source, and whereby said output of said voltage divider is equal to 1/3 of the sum of said second voltage from each phase, said output is thereby proportionately equal to the average rms current in each phase of said three phase AC source. 
     
     
       7. An overload relay with phase current unbalance detection for a polyphase electrical source providing polyphase currents to a load, said overload relay having a trip indication when either an overload condition exists or when one of said polyphase currents exceeds a predetermined percentage of an average of said polyphase currents, said overload relay comprising: a. means to convert each of said polyphase currents supplied to said load from said electrical source to a voltage proportional to said phase current for each of said polyphase currents;   b. means to approximate an rms current equivalent of said phase current from said voltage proportional to said phase current to generate a DC voltage proportional to said rms current for each of said polyphase currents;   c. means to generate an algebraic sum of said DC voltages proportional to said rms current for each of said polyphase currents;   d. means to integrate over time said algebraic sum to generate a trip voltage level, said trip voltage level representative of a composite rms current of polyphase currents;   e. means to compare said trip voltage level with a preset voltage level representing an overload trip setting, said comparison means to indicate an overload condition if said trip voltage level is greater than said preset voltage level;   f. means to generate an average DC voltage from said algebraic sum, said average DC voltage equivalent to said average rms current of said polyphase currents;   g. means to scale said algebraic sum by a predetermined factor representative of an allowable level of phase unbalance; and   h. means to compare said scaled algebraic sum with each of said DC voltages proportional to said rms current for each of said polyphase currents, said comparison means to indicate a phase current unbalance condition if any of said DC voltages for each of said polyphase currents exceeds said scaled algebraic sum.   
     
     
       8. The overload relay with phase current unbalance detection of claim 7 wherein said approximation means of the rms current equivalent of each of said polyphase currents comprises means to generate a first voltage equivalent to an average value of said phase current, means to generate a second voltage equivalent to a peak value of said phase current, and means to sum a fraction of said first voltage with a fraction of said second voltage to provide said DC voltage proportional to said rms current for each of said polyphase currents. 
     
     
       9. The overload relay with phase current unbalance detection of claim 8 wherein said summing means solves an approximation equation to provide said DC voltage proportional to the said rms current for each of said polyphase currents. 
     
     
       10. The overload relay with phase current unbalance detection of claim 9 wherein said approximation equation is equivalent to the equation   I.sub.rms =(0.22*I.sub.pk)+(0.776*I.sub.ave)     wherein I rms  is the rms current of said load, I pk  is the peak current of said load, and I ave  is the average current of said load.   
     
     
       11. The overload relay with phase current unbalance detection of claim 8 wherein said integration means includes means for selecting a trip class, said trip class to determine a trip time by changing a time constant of said integration means. 
     
     
       12. The overload relay with phase current unbalance detection of claim 8 wherein said generation means of said algebraic sum includes means for setting a full load ampere rating of said overload relay, said rating to determine when said overload condition exists. 
     
     
       13. A protective apparatus for controllably interrupting power supplied to an AC load from a polyphase AC source through a switching device, in response to a trip signal, by monitoring phase current in each phase of the polyphase AC source, said protective apparatus comprising: a. approximating means for determining the rms equivalent current of each phase current and generating a proportional voltage level, said voltage level proportional to said rms equivalent current for each phase of the polyphase AC source;   b. averaging means for determining an average rms current that is an average of the rms equivalent currents of all of said phase currents, said averaging means further having means for multiplying said average rms current by a factor representing a percentage of allowable current unbalance in each of said phase currents to generate a scaled trip voltage level;   c. first comparator means for each phase of said polyphase AC source, each of said comparator means having one input coupled to said scaled trip voltage level and another input coupled to said respective proportional voltage level, said comparator means for producing an unbalance signal in response to said proportional voltage level being greater than said scaled trip voltage level;   d. time integration means for producing a pretrip signal, said pretrip signal increasing in magnitude in response to said unbalance signal from at least one first comparator means;   e. second comparator means for comparing said pretrip signal with a predetermined level and for producing said trip signal if said unbalance signal is present for a predetermined time period.   
     
     
       14. The protective apparatus of claim 13 wherein said approximating means includes means to sense actual current in each phase of said polyphase AC source and means to convert said actual current in each phase to a first voltage proportional to said actual current. 
     
     
       15. The protective apparatus of claim 14 wherein said approximating means further includes means to generate a second voltage from said first voltage, said second voltage equivalent to an average value of said phase current, means to generate a third voltage from said first voltage said third voltage equivalent to a peak value of said phase current, and means to sum a fraction of said second voltage with a fraction of said third voltage to provide said voltage level proportional to said rms equivalent current for each phase of the polyphase AC source. 
     
     
       16. The protective apparatus of claim 15 wherein said approximating means solves an approximation equation to provide said proportional voltage level to said rms equivalent current for each phase of the polyphase AC source, said approximation equation is equivalent to the equation   I.sub.rms =(0.22*I.sub.pk)+(0.776*I.sub.ave)     wherein I rms  is the rms current of said load, I pk  is the peak current of said load, and I ave  is the average current of said load.   
     
     
       17. The protective apparatus of claim 15 wherein said averaging means includes an operational amplifier having an adjustable resistor in a feedback path between the amplifier output and its inverting input, said resistor for determining gain of the amplifier, said gain to scale said average value of said phase current by said factor representative of an allowable level of phase current unbalance between each phase of said polyphase AC source. 
     
     
       18. The protective apparatus of claim 17 wherein said averaging means further includes a voltage divider between an output of said operational amplifier and circuit common, said voltage divider comprising two series connected resistors having a resistance ratio of 2:1 for a three phase AC source, and whereby said output of said voltage divider is equal to 1/3 of the sum of said second voltage from each phase, said output is thereby proportionately equal to the average rms current in each phase of said three phase AC source. 
     
     
       19. The protective apparatus of claim 13 wherein said polyphase AC source is a three phase AC source.